PLL Hum Cancellation Circuit
This report describes the operation and adjustment of a Phase Locked Loop hum cancellation circuit that can be used to reduce the residual hum from any amplifier. Normally this is useful for DHT amplifiers with AC operated filaments, as even with a hum balancing potentiometer, there is a residual hum component at twice the line frequency that is unavoidable, even with perfectly filtered HT voltage. However, it may be applied to any amplifier.
I've reported a hum cancellation circuit (here) that provided improvement in hum performance of many DHT circuits. Some people have used this successfully, some with only limited success. The key is to get the amplitude and phase set just right to cancel the residual hum component. This is not always easy nor convenient, so I sought a way to make this concept more universally applicable. This report describes the results.
Theory of Operation
The key behind this version is the use of a phase locked loop, fed from the mains supply (50 or 60 Hz). A PLL consists of a phase comparator, a loop filter and a voltage controlled oscillator (VCO). The phase comparator compares the phase and frequency of an incoming signal (the line frequency, not the "music" source) against an oscillator, and adjusts the oscillator frequency so that it equals the incoming reference.
In this case, the VCO operates at 16 times the line frequency (800 or 960 Hz), which is divided by 16 and compared to the line frequency. The x16 allows the line to be split into 16 equal portions. For simplicity, I opted to split this into 8 equal portions, each of which separately control a trimmer pot. The portions are summed and filtered to produce a cancellation signal that can be used to compensate for an arbitrary phase and amplitude of the residual hm signal to be canceled. The filter removes any higher order components, so that more audible, but lower amplitude higher order components are not introduced into the amplifier.
The "injection" into the main amplifier may be at any convenient point. In the 845 amp, it is introduced into the otherwise "unused" grid of the upper tube of the cascode stage. It may also be introduced into the output tubes bias path. For details on how to do this, see the first hum balancing report. (here).
Set up and Adjustment
A scope definitely helps!
First build the circuit, and attach the inputs (upper left in the schematic) to a 6.3 volt source. This may be a 6.3V winding with one side grounded, or a 6.3 volt CT winding where the center tap is grounded. If the latter, the attachment is one side (either side) of the 6.3 and ground. (This will place 3.15VAC on the circuit which is sufficient).
The first adjustment is to set the VCO frequency, shown in the schematic as "lock PLL". This will be near minimum resistance with 60Hz mains, and near maximum resistance with 50Hz mains. If you have a scope, you will see that pin 3 of the '4046 is at the same frequency as the mains.
The actual balancing is independent for each of the 2 amplifier channels.
A main level control and a set of 8 amplitude-phase controls is used for the channel. First, set one of the 8 A-P controls at one end of the control, and the other 7 at the other end of the control, and the main level control about "half way up". If you have a scope, you should be able to see a sort of peaky signal at line frequency at the output. (hum bal ch1 or hum bal ch2). If you don't have a scope, you should be able to see a few hundred millivolts of signal on an AC voltmeter.
Now turn the main level control all the way down, and connect it into your amplifier circuit. Then turn on the amp and slowly advance the main control. The hum at the amplifier output should either go up (according to Murphy's law, it will go up) or down. Continue advancing the control until the hum is about 6 dB stronger (or 6dB weaker) than it was without the system connected.
Now, sequentially adjust the 8 A-P controls for minimum hum. You will have to repeat this sequence a few times. If you get to the point where you want more range on the A-P pots, simply increase the main control and repeat the sequence. A scope is definitely a help, as it allows you to see the effect of each A-P control on the overall hum component, allowing you to quickly find an optimum.
After you have one channel set up, repeat the procedure with the other set of controls on the other channel. There is essentially no interaction between channels.